This keyword was added in TOSCA 1.3, in section 3.8.2.1.
We've discussed it several times in the ad hoc meetings, in relation to ... many topics, most notably the instance model. But it remains for me a feature I simply cannot implement in Puccini.
In Puccini, "occurrences" in requirement definitions are interpreted to mean exactly how many times the requirement can/must be assigned in the node template, with the additional understanding that the sequenced list notation is there exactly in order to allow multiple assignments. I think this interpretation makes sense in many practical ways. Note that this has nothing to do with the instance model: this is all about TOSCA grammar and how many times you are allowed to specify something. (Puccini uses a similar design-time interpretation for "occurrences" in capabilities, for which it specifies how many times the capability can/must satisfy a requirement.)
But, with that understanding, I have no idea what "occurrences" could mean in the requirement assignment. It seems the thinking here is that "occurrences" has something to do with the instance model. So, for now, Puccini parses this keyword as proper syntax, but never does anything with it.
Indeed, for TOSCA 2.0 I've suggested several times that we should remove this keyword everywhere. If it's a design-time feature then it's rather narrow and hard to imagine as being very useful (and should probably be renamed to "assignments"). If it's a runtime feature, referring to the instance model, well that's a whole can of worms that we've discussed at length. I'll likely continue to argue that TOSCA should not have an opinion on the structure, and if it does then a simple counting of "how many runtime relationships" seems woefully insufficient.
But, I do want to open this up more on the email list, perhaps someone can shed some light on the confusion? Also, what do you think I should do in Puccini if it's impossible to "support" this keyword in requirement assignments?